Ok, Ive had my car since '02. I bought it with 126k miles and have since taken that to upwards of 300k+ (odometer stopped working at 225k about 4 years ago).

So no surprise I'm in the market for a new engine/short block to replace mine when it eventually fails. Originally i was like many others looking to to a forged 4.3L M112 build, but have come to realize that I would never use it to it's potential. However I would like a bump in displacement. So I've been eyeing the F-150 4.2L as a possible/less expensive replacement.

I can get used 4.2's for $600-$1000, but I've also found a site where I can get a rebuilt 4.2 short block for $1200... which is what i think I'll go for.

Now most of us look to power adders to get us to a competitive performance level, it only makes sense, blower or turbo there are always extensive factors to consider, one being compression ratios and how much boost is safe.

The 4.2L's have a compression ratio of 9.2:1, 9.26:1, or 9.3:1, depending on the year, while the 3.8L is 9.0:1 across the board if I remember correctly. Now here's my question...

If I were to get the rebuilt 4.2L with say 9.2:1 compression, how much boost can be run safely from either an intercooled P-1SC centri blower or single 62mm turbo...just for example?

 

On a stock 4.2L bottom end, you'll peak out about 375 -400 hp safely. Lower compression is more boost friendly, but the differences between those three is not enough to really make a HUGE difference until you start looking into really big power.

 

That'll do just fine. My goals for a v6 build since the beginning have always been for 400 bhp. So ballpark... 6-8psi max? or even less?

 

Try more, 400rwhp is going to require somewhere around 11psi from a turbo and somewhere around 13-14 from a procharger kit.

And your gonna need cam, ported heads intakes, etc.

 

I understand the need for an upgraded cam and ported heads. Once i pick up the new short block i was going to replace the cam and once my 3.8 decides to give it up I'll have the heads ported locally before having them swapped over.

 

if you want boost may as well save for forged.

 

Those extra few thousand dollars are much harder for me to come by these days. Doing a Boosted mild build falls much better into what I can afford to spend of my "play around" cash.

 

Then stay around 350rwhp.

 

That's why I said 400 bhp and not rwhp.

 

So anyway; just so I can feel like I've gotten a clear answer to my original question... how many lbs of inter-cooled boost is safe for a non-forged 9.3:1 compression ratio?